The story roughly goes like this: after his defeat at Waterloo in June 1815, Napoleon gave himself up to an English ship on the coast of France, naively thinking he would be able to while away the rest of his days in the style of a country squire.

The English government had other plans. Such was its hatred and fear, it exiled Napoleon to one of the remotest regions in the world, the island of St Helena in the South Atlantic, midway between the coasts of Africa and South America. On his arrival on St Helena, Napoleon's appointed residence, a rundown old farmhouse called Longwood, had yet to be renovated and refurbished. While waiting, he chose to remain in a small pavilion near what was referred to as The Briars, a house occupied by the Balcombe family.

Thus are lives thrown together, the great and the small inextricably intertwined, to be played out on the stage of history. For the next two months, the Balcombes lived cheek-by-jowl with the fallen Emperor. One of the Balcombes was a precocious, somewhat untamed 13-year-old, Lucia Elizabeth, known as Betsy, who developed a teasing and playful relationship with Napoleon. By all accounts Betsy was, in the words of Tom Keneally, "a flouter of mean authority", offended when "the paltry dominated over the august".

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We know much of this because Betsy wrote her memoirs some 30 years later in an attempt to make a little money off the celebrity surrounding Napoleon when she had fallen on relatively hard times. Her story – and we really only have her word for most of it - has subsequently inspired more interest than Betsy probably merits. She was, after all, a fairly insignificant character, one who had no perceptible impact or influence on the great man. Why then this interest in the girl?

As both the biography by Anne Whitehead and the novel by Keneally demonstrate, Betsy renders Napoleon human. She brings out a side of him that people are not familiar with: childlike, charming, happy, even vulnerable. She did so by not succumbing to the aura of power that surrounded Napoleon. Rather, she behaved like a cheeky, even disrespectful child who was supposedly abrupt on occasion, called Napoleon by his English nickname, "Boney", and challenged him when others dared not.

Betsy on St Helena is the starting point of two different but not entirely dissimilar works focused on the improbable relationship between the teenager and the former ruler of Europe. Whitehead uses the girl as a prism through which the reader views four entirely different societies – St Helena in the first stages of Napoleon's exile; Georgian England; Restoration France; and colonial New South Wales in the 1820s – all places where Betsy and her family resided at various stages of their lives.

In other words, Whitehead's biography is not always about Betsy, partly because we don't know all that much about her. Her father, William Balcombe, a portly, jovial, naïve character, figures more prominently in parts of the book than his daughter. Whitehead's book is less biography than a history of the world through which Betsy travelled, as well as some of the places the author has travelled to in the writing of the book, deftly woven into the narrative.

Keneally, on the other hand, uses history to tell an imaginary tale of the relationships on the island prison not only between Napoleon and Betsy, but also between the other people who shared his exile. Those familiar with Keneally will recognise his approach to fiction: "Telling the truth by telling divine lies", as he has put it in a number of interviews.

In doing so, he succeeds, with touches of brilliance, in bringing to life characters in more detail than history ever possibly could. For it is not just a story about Betsy, it is also a coming-of-age story, one in which the protagonist gradually becomes aware of the foibles of human nature. Through her we discover an adult world, a world that she constantly grapples to come to terms with.

Whitehead's history also underscores the difficulties of a woman living in the first half of the 19th century. After leaving St Helena, she eventually married Edward Abell, who turned out to be a scoundrel, and who abandoned her and their baby daughter. He probably only married her in the hope of gaining something out of the father who, it was rumoured, was the illegitimate son of the Prince Regent. Edward later turned up in Sydney trying to cash in on William Balcombe's position as colonial treasurer, and had to be bought off. But the situation for Betsy was always fraught; legally Edward could dispose of any wealth or property she accumulated. Whitehead's history is thorough, possibly the best available account of Betsy, interesting for what it tells us about the life of an ordinary woman.

Keneally's novel is focused on St Helena, with a longish postscript about Betsy's life after she leaves the island. It largely sticks to the script. That is, much of the story more or less took place as he describes it, except for an unexpected twist towards the end of the novel. It is meant to be a "getting of wisdom" moment when Betsy discovers her mother taking part in a sort of orgy, but for me it was more of a guffaw moment, less shocking than disrespectful to the memory of those involved. Apart from this peculiar development, Keneally is more constrained by history, rather than using it to set the story free.

I am thinking of something such as Simon Leys' The Death of Napoleon, a novella brilliant in its simplicity, both in language and plot. In it, Napoleon escapes St Helena, works as a ship's hand, visits the battlefield of Waterloo as a tourist, and makes his way to Paris, where he discovers that the Napoleon-lookalike he left behind on St Helena has died. An attempt to reveal his true identity sees him being threatened with internment in an insane asylum.

Leys' book can be read as a parable on the vanity of all human ambition. Keneally is a consummate wordsmith but I am not entirely sure what the purpose of this novel is, except to entertain. It might be unfair comparison, but it doesn't soar to the heights of a Leys. Both Whitehead and Keneally have made the voyage to St Helena in the course of their research, lending their writing a knowledge and understanding that might otherwise have been lacking. They are both enjoyable reads in their own way.

One cannot help but grow fond of Betsy, despite or because of all her imperfections. Ultimately though the lingering question for me is not so much whether Whitehead and Keneally have achieved what they set out to do, nor how well they may have achieved it, but whether Betsy was worthy of their attentions in the first place. Only the reader's sense of satisfaction can answer that.

Philip Dwyer is director of the Centre for the History of Violence at the University of Newcastle and the author of Napoleon: The Path to Power (2007) and Citizen Emperor: Napoleon in Power (2013). He is writing about Napoleon's exile on St Helena.